The present invention relates to a battery case used as an exterior case for various kinds of batteries, such as an alkaline dry battery, a nickel-cadmium rechargeable battery, and a lithium rechargeable battery, and to a method of manufacturing such a battery case with DI (drawing processing and ironing processing) processing.
Conventionally, a method for manufacturing a battery case chiefly adopts the transfer drawing method for manufacturing a battery case of a predetermined shape by repeating deep-drawing processing and punching processing for 10 to 13 steps by a transfer press machine, and the DI processing method for manufacturing a cup-shaped intermediate product in a deep-drawing step by a press machine first and thence manufacturing a battery case of a predetermined shape from the cup-shaped intermediate product in a drawing step using a drawing die and in an ironing step using an ironing die. The DI processing method is advantageous over the transfer drawing method in that the productivity can be enhanced by reducing the number of steps, and because only the side wall is stretched to reduce the thickness thereof during the step of applying the ironing processing to the cup-shaped intermediate product, a larger capacity can be achieved, which makes it possible to enhance the battery characteristics with an increasing capacity resulted from an increased filling agent while reducing the weight, and therefore, the utilization factor thereof has been rising.
The battery case manufactured by the DI processing method has many advantages as described above; however, on the other hand, the inside face of the side wall of the battery case is pressed hard against the body portion of the punch during the ironing processing, and the inner surface of the side wall is undesirably made smooth. As a result, there arises a problem that a contact area between the inside face of the side wall and an active material or a cathode mix accommodated in the battery case is diminished and an internal resistance of the battery increases, which results in deterioration of the battery characteristics. In particular, an alkaline dry battery establishes electrical conduction at the positive electrode side only by a secondary contact between the inside face of the side wall of the battery case serving also as the positive electrode and a pellet of the cathode mix, and for this reason, an increased internal resistance of the battery caused by the aforementioned diminished contact area lowers the battery performance drastically, and among others, the high load discharge characteristics for drawing a large current are deteriorated.
Accordingly, an internal resistance of the battery between the inside face of the side wall and the cathode mix or the active material is reduced conventionally by applying a conductive coating material, such as carbon, or a conductive agent onto the inside face of the side wall of the battery case, or by forming a nickel-tin plated alloy layer on the inside face of the side wall. Theses means, however, cannot reduce the internal resistance of the battery sufficiently; moreover, because the surface on the inside face of the side wall is smooth, the retentivity for the conductive coating material or the conductive agent is too weak to achieve required post-preservation battery characteristics.
On the other hand, a battery case manufactured by a processing method other than the above-described DI processing method, for example, the above-described transfer drawing method, has the surface on the inside face of the side wall made rough with minute wrinkles made while the drawing step is repeated a large number of times. Hence, a contact area with the cathode mix or the active material is enlarged, which makes it possible to suppress an increase in internal resistance of the battery. However, the ironing processing is seldom applied in the transfer drawing method, and in that event, the thickness of the side wall is hardly reduced in comparison with the thickness of the bottom wall. Hence, the capacity of the battery case remains the same, and there is a drawback that a filling amount of the active material or the like is reduced, and the charge and discharge characteristics are deteriorated. In addition, because this manufacturing method of the battery case has too many steps as described above, the productivity cannot be enhanced, which results in an increase of the manufacturing costs.
In light of the conventional problems as described above, the present invention has an object to provide a battery case having a shape that enlarges a contact area between the inside face of the side wall and the cathode mix or the active material while keeping a large capacity, and a method for manufacturing such a battery case at high productivity.
In order to achieve the above objects, a battery case of the present invention is formed in such a manner that: a thickness t1 of a side wall with respect to a thickness to of a bottom wall satisfies t1 =xcex1t0 (xcex1=0.2 to 0.7) by undergoing an ironing step using ironing dies aligned in multiple stages; and an inside face of the side wall has a rough surface having average surface roughness of 0.2 xcexcm to 2.0 xcexcm by undergoing a drawing step after the ironing step.
This battery case has a large contact area between the inside face of the side wall and the cathode mix or the active material accommodated therein, whereby an internal resistance of the battery is reduced drastically. Also, when a conductive material, such as carbon, is applied onto the inside face of the side wall, the retentivity for the conductive material or the like is enhanced, which makes it possible to maintain high post-preservation battery characteristics over a long period. In order to enlarge the contact area, the average surface roughness is set to a range from 0.2 xcexcm to 2.0 xcexcm. Also, this battery case has a large capacity, because the side wall is made thinner than the bottom wall by undergoing the ironing step. Hence, a filling quantity of the cathode mix or the active material is increased, and the battery performance, such as charge and discharge characteristics, is enhanced.
A manufacturing method of a battery case of the present invention includes: a first step of manufacturing a battery case element by applying drawing processing by at least one drawing die and ironing processing by ironing dies aligned in multiple stages to a cup-shaped intermediate product, so that a thickness t1 of a side wall with respect to a thickness t0 of a bottom wall satisfies t1 =xcex1t0 (xcex1=0.2 to 0.7); and a second step of manufacturing a battery case by reducing an outside diameter of the battery case element to a predetermined outside diameter without changing the thickness of the side wall by applying drawing processing with drawing dies aligned in multiple stages.
According to this manufacturing method of the battery case, ironing processing is not applied in the second step. Hence, the battery case element is plastically deformed to a condition that the outside diameter is reduced to a predetermined outside diameter while maintaining the thickness of the side wall. Accordingly, the inside face of the side wall of the battery case is made rough with minute wrinkles made while the outside diameter is reduced without changing the thickness of the side wall. Consequently, fine irregularities are formed across the inside face at a uniform density, which enlarges a contact area with the cathode mix or the active material. The inner surface of the side wall is made rough by undergoing a series of manufacturing procedures of the battery case without requiring any special step. Hence, the productivity is not impaired.
Also, because the ironing processing is applied to the cup-shaped intermediate product in the first step, the side wall is made thinner than the bottom wall, and the outside diameter is reduced without changing the thickness of the side wall in the second step. Hence, the completed battery case has a large capacity. Further, in the second step, a material corresponding to deformation caused by the diameter reduction of the battery case element is flown so that it is released into the bottom wall. Hence, a step portion is formed at the peripheral portion of the bottom wall thicker than the side wall. As a result, the completed battery case has the strength high enough to prevent an occurrence of buckling or the like.